Method for optimizing skin cooling level of an occupant support surface

ABSTRACT

A method and apparatus for withdrawing heat from a surface for supporting a person including determining a vasoconstriction threshold for the person and operating the apparatus to maintain the rate of heat withdrawal below the vasoconstriction threshold for the patient.

PRIORITY CLAIM

This application claims priority under 35 U.S.C. § 119(e) to U.S. Provisional Application No. 62/802,920, filed Feb. 8, 2019, which is expressly incorporated by reference herein.

TECHNICAL FIELD

The present disclosure is related to bed mattresses for supporting patients. More specifically, the present disclosure is related to a coverlet for hospital beds, medical beds, or other types of beds where the coverlet includes structure that allows for the control of heat withdrawal from a surface supporting a patient. More specifically, the present disclosure relates to the operations of an air supply system to determine an optimal heat withdrawal rate for a particular patient and control air being delivered to the coverlet to maintain the optimal heat withdrawal rate.

BACKGROUND

In a care facility, such as a hospital or a nursing home, patients are often placed on patient support apparatuses for an extended period of time. Patients who are positioned on the patient support apparatus often have a risk of developing certain skin condition, such as bed sores (also known as pressure sores or decubitus ulcers), due to heat and moisture along the interface of the patient with the surface of the bed mattress. In an effort to mitigate or prevent such conditions, some bed mattresses have a built-in microclimate structure. The microclimate structure may conduct air along the interface of a patient with the surface to keep the patient's skin cool and dry. Some microclimate structures require a large volume of air to be supplied to them in order to provide an effective amount of cooling and drying to a patient's skin.

SUMMARY

The present disclosure includes one or more of the features recited in the appended claims and/or the following features which, alone or in any combination, may comprise patentable subject matter.

According to a first aspect of the present disclosure, a method for controlling performance of a heat withdrawal coverlet for removing heat from a surface supporting a person that has a flow path for guiding a stream of air along at least a portion of the surface comprises measuring heat withdrawal rate of the coverlet over time, identifying when the coverlet is withdrawing heat at first rate, increasing the heat withdrawal rate from the first rate between in predefined increments, and measuring a skin temperature of an occupant at each increment of increase of the heat withdrawal rate. The method also include recording the skin temperature of the occupant at each increment of increase of the heat withdrawal rate, comparing the skin temperature of the occupant and the heat withdrawal rate increment to determine a slope of the relationship between the skin temperature of the occupant and the heat withdrawal rate, and determining the skin temperature and the heat withdrawal rate at which the slope breaks in linearity to determine a vasoconstriction threshold.

In some embodiments, the relationship between the skin temperature of the occupant and the heat withdrawal rate is an inverse relationship such that as the heat withdrawal rate decreases, the skin temperature of the occupant increases.

In some embodiments, an optimal heat withdrawal rate is located prior to a slope break, the optimal heat withdrawal rate configured to maximize the heat withdrawal from the occupant.

In some embodiments, the optimal heat withdrawal rate determines the rate at which heat is withdrawn from the skin of an occupant prior to vasoconstriction.

In some embodiments, the slope decreases after breaking such that the measurement of the slope before the break is greater than the measurement of the slope after the break.

In some embodiments, the method further comprises decreasing the heat withdrawal rate after identifying the vasoconstriction threshold, waiting an amount of time so that the skin temperature of the occupant decreases and returns to a baseline temperature, identifying when the coverlet is withdrawing heat at a predefined rate, incrementally increasing the heat withdrawal rate from the predefined rate until the optimal heat withdrawal rate is reached, and maintaining the optimal heat withdrawal rate.

According to a second aspect of the present disclosure, a method for controlling performance of a heat withdrawal coverlet for removing heat from a surface supporting a person having a flow path for guiding a stream of air along at least a portion of the surface comprises measuring the skin temperature of a person on the coverlet, varying the operation of the coverlet to vary the heat withdrawal rate, monitoring the slope of the rate of change in skin temperature to the heat withdrawal to determine a change in linearity of the slope, using the heat withdrawal rate at the change in linearity to determine vasoconstriction threshold for the person, and maintaining the heat withdrawal rate of the coverlet to operate below the vasoconstriction threshold of the person.

According to a third aspect of the present disclosure, an apparatus for controlling the heat withdrawal from a patient's skin comprises a coverlet having an upper surface that is vapor permeable and air impermeable, the coverlet have inlet and an outlet and an interior space that provides a flow path for air to flow from the inlet to the outlet, the coverlet including a sensor for monitoring the heat withdrawal from the coverlet and a sensor for measuring the temperature of the skin of a person supported on the upper surface of the coverlet, an air treatment system having an inlet for admitting ambient air and an outlet for discharging treated air, a conduit connecting the outlet of the air treatment system to the inlet of the coverlet, and a controller. The controller includes a processor and a memory device. The memory device includes instructions that, when executed by the processor, cause the controller to monitor the sensors and operate the air treatment system to vary the heat withdrawal from the coverlet and monitor the skin temperature of the person to determine the slope of the rate of change in skin temperature to the heat withdrawal to determine a change in linearity of the slope, use the heat withdrawal rate at the change in linearity to determine vasoconstriction threshold for the person, and maintain the heat withdrawal rate of the coverlet to operate below the vasoconstriction threshold of the person.

In some embodiments, the air treatment system includes a blower.

In some embodiments, the air treatment system includes a cooler for cooling the ambient air.

In some embodiments, the air treatment system includes a heater for heating the ambient air.

In some embodiments, the air treatment system includes a water removal system for removing water from the ambient air.

In some embodiments, the air treatment system includes a valve assembly for controlling the flow of the air out of the air treatment system.

Additional features, which alone or in combination with any other feature(s), such as those listed above and/or those listed in the claims, can comprise patentable subject matter and will become apparent to those skilled in the art upon consideration of the following detailed description of various embodiments exemplifying the best mode of carrying out the embodiments as presently perceived.

BRIEF DESCRIPTION OF THE DRAWINGS

The detailed description particularly refers to the accompanying figures in which:

FIG. 1 is a perspective view of an patient support system having an coverlet including a structure for permitting heat to be withdrawn from a surface supporting a patient;

FIG. 2 is a diagrammatic view of the patient support system with the coverlet supported on an patient support apparatus and showing a fluid supply assembly in fluid communication with the occupant support surface;

FIG. 3 is a diagrammatic cross-sectional view of the coverlet of FIG. 1 positioned on a non-powered mattress and the coverlet;

FIG. 4 is a diagrammatic cross-sectional view of the coverlet of FIG. 1 showing the communication of heat and moisture through a top layer and a bottom layer of the coverlet and the flow of air through the coverlet;

FIG. 5 is a diagrammatic view of the air supply assembly of FIGS. 1 and 2;

FIG. 6 is a flow chart for the control of the air supply system to control heat withdrawal from a patient's skin;

FIG. 7 is a graph showing skin temperature as a function of surface heat withdrawal and an inverse relationship between skin temperature and surface heat withdrawal;

FIG. 8 is a perspective view of a dehumidification device comprising an array of vertically oriented fibers; and

FIG. 9 is a perspective view of a user interface for the operation of an air supply system to control the heat withdrawal from the coverlet.

DETAILED DESCRIPTION

A patient support system 10 according to one illustrative embodiment of the current disclosure is shown in FIG. 1. The patient support system 10 includes a patient support apparatus 12, illustratively embodied as a hospital bed, and a patient support surface 14 supported on the patient support apparatus 12. The patient support system 10 further includes an air treatment system 16 that provides air to the patient support surface 14 (see FIG. 2). The patient support apparatus 12 may also be embodied as a stretcher, an operating room table, a wheel chair, or other person supporting structure known in the art. An illustrative patient support apparatus 12 embodied as a hospital bed is shown in FIG. 1.

The patient support apparatus 12 includes a base frame 18 and an upper frame 22 supported on a lift system 20. The lift system 20 is operable to raise and lower the upper frame 22 relative to the base frame 18 and to tilt the upper frame 22 relative to the base frame 18 to achieve Trendelenburg and reverse-Trendelenburg positions as known in the art. The upper frame 22 supports a deck 38 that is movable to multiple positions as is known in the art. In the illustrative embodiment, the patient support surface 14 includes a coverlet 26 attached to and supported by a mattress 28. The patient support surface 14 is configured to support a person thereon and move with the deck 38 between the various configurations. The patient support surface 14 includes a calf section 28, a thigh section 30, a seat section 32, and a head and torso section 36 as shown in FIG. 2, which are supported on corresponding sections of the deck 38. In the illustrative embodiment, the patient support surface 14 is a non-powered (static) surface. In another embodiments, the patient support surface 14 which is configured to receive an input, such as, fluid from a fluid supply that can alter a characteristic of the patient support surface 14. The patient support surface 14 may include a non-powered mattress 28 and a coverlet 26 positioned on the mattress 28 as shown in FIG. 1.

The coverlet 26 includes a heat withdrawal structure 44, a top layer 46, a bottom layer 48, an inlet 50, an outlet 52, and a 3-dimensionally engineered spacer 54 as shown in FIGS. 3 and 4. The top layer 46 and the bottom layer 48 are coupled together along their edges to form an inner chamber 56. In the illustrative embodiment, the edges of the top layer 46 and the bottom layer 48 are coupled together using RF welding technology. The top layer 46 and the bottom layer 48 are both configured to be vapor permeable and air impermeable. This configuration prevents air passing through the coverlet 26 from impinging on the skin of a person positioned on the coverlet 26 while allowing the moisture produced by the person to pass through the top layer 46 and be exhausted with the air passing through the coverlet 26 out the outlet 52.

The 3-dimension engineered spacer 54 is positioned within the inner chamber 56 and is air and moisture permeable. The 3-dimensional engineered spacer 54 maintains a path for air to flow through when a person is supported on the coverlet 26. In the illustrative embodiment, the 3-dimensionally engineered spacer 54 is Spacenet®. The 3-dimensionally engineered spacer 54 is positioned within an inner chamber 56 and is configured to be air and moisture permeable. The inlet 50 and outlet 52 are generally located on opposite ends of the coverlet 26 and allow a fluid, such as air, to be communicated into the inner chamber 56 of the coverlet 26, and to be exhausted from the coverlet 26, respectively, as shown in FIGS. 2 and 4.

Referring to FIGS. 2 and 4, the heat withdrawal structure 44 is configured to receive air from the air treatment system 16 and conduct treated air flow 74 through the inner chamber 56 of the coverlet 26 to cool and dry the interface between a patient and patient support apparatus 12 to promote skin health by removing patient heat and moisture along the interface when the patient is supported on the patient support apparatus 12. Heat withdrawal structure 44 generally spans laterally from a left side 60 to a right side 62 and extends longitudinally along the length of the coverlet 26 from a foot end 64 to a head end 66. In some embodiments, the heat withdrawal structure 44 may be configured us that less than portion of the coverlet 26 so that air flow in the heat withdrawal structure 44 is directed to a specific region of the body of a person supported on the coverlet 26.

The coverlet 26 further includes a plurality of sensors 24 in electrical communication with a controller 80. The sensors 24 are configured to measure the temperature, moisture, humidity, and/or heat withdrawal levels of the occupant and detect the presence of liquid on the coverlet 26 so that the heat withdrawal structure 44 may record and store such measurements in the controller 80 such that a caretaker may access the measurements. If the sensors 24 detect a level exceeding a predetermined threshold level, the controller 80 is configured to automatically stop the flow of air from the air treatment system 16 by closing the inlet 50. The cessation of air prevents over or under drying of the patient's skin.

Referring to FIG. 2, the air treatment system 16 illustratively includes a blower 84, a cooler 86, a water collection system 88, and a heater 90. The controller 80 is electrically coupled to the blower 84, the cooler 86, the water collection system 88, and the heater 90 for electronic communication and control. The blower 84 is configured to draw in ambient air and is arranged upstream of the heater 90. The heater 90 is arranged in line with the blower 84 and configured to warm ambient air produced by the blower 84 prior to the air being delivered to the heat withdrawal structure 44. The cooler 86 or other air conditioning device(s) may also be positioned between the blower 84 and the heat withdrawal structure 44 and configured to prepare the air for use in a therapeutic flow adjacent to the patient's skin. The sensors 24 may also be placed elsewhere in the air flow 74 and configured to provide feedback to the controller 80 or user interface 82. In other embodiments, the air treatment system 16 may pressurize ambient air prior to delivering the air to the heat withdrawal structure 44.

As shown in FIG. 2, the illustrative heat withdrawal structure 44 is configured to receive air from the air treatment system 16 which is mounted on the base frame 18, but in other embodiments, the air treatment system 16 may be integrated into the upper frame 22 of the patient support apparatus 12. When the air treatment system 16 is integrated into the base frame 18, the functions of the controller 80 may be placed on a footboard 68 or a siderail 70 of the patient support apparatus 12. The air from air treatment system 16 is introduced into the heat withdrawal structure 44 at the inlet port 50 located at the foot end 64 configured to flow through the inner chamber 56 of the coverlet 26 toward the head end 66 of the coverlet 26 as suggested by arrows 74 in FIG. 2. The air flows towards the outlet 52 positioned at the head end 66 of coverlet 26 to be exhausted from the heat withdrawal structure 44.

The controller 80 is in communication with the air treatment system 16 and configured to receive input either automatically via the plurality of sensors 24, manually via the user interface 82, or some combination thereof. The input is then conveyed to the controller 80 via electronic communication such that the controller 80 is configured to wirelessly communicate with the sensors 24, the user interface 82, the air treatment system 16, or some combination thereof.

Manual input may be accomplished by the user through the user interface 82. The user interface 82 includes a display screen 94 and a plurality of buttons 96 for inputting patient information and/or controlling operation of the air treatment system 16 and patient support surface 14. Particularly, the controller 80 allows a user to adjust the air flow 74 provided by the air treatment system 16 to the coverlet 26 and, in some embodiments, to additionally adjust the temperature of the air provided by the air treatment system 16 to the coverlet 26. Specifically, in some embodiments, the controller 80 may include a patient information input panel, an alarm panel, a lateral rotation therapy panel, an inflation mode panel, a normal inflation control panel, a microclimate control panel, or some combination thereof. The controller 80 is configured to regulate the operation the air treatment system 16 and direct the flow of air created therein. Illustratively, the controller 80 is coupled for communication with a valve box 92 to control the rate by which treated air flows through the heat withdrawal structure 44.

The controller 80 comprises at least one processor 85 and at least one memory device 87. The memory device 87 is configured to store instructions for execution by the processor 85. The controller 80 is further configured to receive information from the sensors 24 and user interface 82 (via electronic communication as discussed above) as inputs to the processor 85 in executing the instructions stored in the memory device 87. The controller 80 is further enabled to communicate information as outputs signals to the air treatment system 16, the fluid control valve box (not shown), the other components of the patient support apparatus 12, or some combination thereof in order to control the operation of the heat withdrawal structure 44. Illustratively, the controller 80 is configured to wirelessly communicate with the sensors 24 and the user interface 82 as shown in FIG. 5, but could be a wired connection in other embodiments.

The controller 80 is further configured to individually control the cooler 86, the heater 90, the blower 84, or any combination thereof. As mentioned above, the conduit 98 is configured to facilitate communication of air between the air treatment system 16 and the coverlet 26 as shown in FIG. 2. In one illustrative embodiment, an the conduit 98 is removeably coupled to the outlet 58 of the air treatment system 16. The air flow is directed from the inlet 50 through the inner chamber 56 of the coverlet 26 and exhausted from an outlet 52 formed in the coverlet 26. As fluid passes through the coverlet 26, heat and moisture communicated through the top layer 46 and/or bottom layer 48 is absorbed by the air flow 74 and exhausted with the air flow 74 from the outlet 52. Such absorption and exhaustion occurs due to the transfer of heat from the patient to the heat withdrawal structure 44.

Two principal mechanisms of heat transfer affect the operation of the heat withdrawal structure 44: dry heat transfer and wet heat transfer. Dry heat transfer is proportional to temperature difference and is independent of the presence or absence of liquid phase perspiration at the occupant/surface interface. The potential of the coverlet 26 to effect dry heat transfer at a given temperature difference is referred to as its dry flux capacity (DFC). The dry heat transfer actually realized during operation of the system described herein is the actual dry flux (DF).

The second mechanism of heat transfer, wet heat transfer, is proportional to the difference in the partial pressure of water vapor (perspiration) at the occupant's skin and the partial pressure of water vapor in the air flow 74. The potential of the support surface to affect wet heat transfer is its wet flux capacity (WFC). The wet heat transfer actually realized during operation of the system described herein is the actual wet flux (WF).

To optimize heat withdrawal and patient cooling, a method, as shown in FIG. 8, determines the uppermost level of cooling tolerable by a patient before vasoconstriction of the blood vessels occurs. The blood vessels are configured to provide blood to the occupant's skin, but vasoconstriction halts this process. Vasoconstriction is the contraction of blood vessels resulting in increased blood pressure and may be affected by hemodynamic determinants, such as blood pressure, blood flow, vascular tone, and velocity of blood flow, environmental determinants, such as increased cooling of the skin, or any combination thereof. Vasoconstriction of the blood vessels leads to decreased tissue perfusion such that the blood flow through the tissue decreases thereby leading to a disruption in the exchange of gases between the blood and the cells of the capillary bed resulting in ischemia. Ischemia results in the skin tissue failing to receive adequate oxygen supply which further causes complications such as cardiovascular disease and a plethora of other conditions.

Ischemia (aka: poor perfusion, malperfusion) is linked to various health problems and lead to poor thermoregulation of an occupant due to the lack of blood flow reaching the skin cells of the patient. The lack of blood flow not only decreases the amount of oxygen the skin cells receive from the blood but also increases the occupant's skin temperature. Blood flow to skin cells dissipates the heat of an occupant by redirecting warm blood closer to the surface of the skin so that it may help cool an occupant through perspiration and thermal dissipation. Therefore, as shown in FIG. 9, an occupant's skin temperature (° F.) and the surface heat withdrawal (w/m²) are inversely related such that an increase in surface heat withdrawal leads to a decrease in occupant skin temperature. It has been determined that this relationship is described by the following equation:

Skin T(° F.)=−0.0697*HW+100.01

wherein HW represents surface heat withdrawal.

Operation of the heat withdrawal structure 44 can be understood by referring to the graph of FIG. 9 showing skin temperature, expressed in degrees Fahrenheit (° F.), as a function of surface heat withdrawal, expressed in watts (w/m²) in conjunction with the equation: SkinT (F)=−0.0697*HW+100.01. From left to right, the circles on the graph represent the mean skin temperature of occupants experiencing heat withdrawal rates of 13 w/m², 29 w/m², 63 w/m², 73 w/m², and 168 w/m², respectively. Line RHW represents the rate of surface heat withdrawal per degree Fahrenheit of an occupant's skin temperature and is shown as a relatively linear slope. As evidenced in the graph, as surface heat withdrawal increases and skin temperature decreases, line RHW breaks in linearity. This break identifies the vasoconstriction threshold of an occupant and may be used to provide optimal surface heat withdrawal resulting in the maintenance of a comfortable body temperature and tissue perfusion of the patient.

To determine the vasoconstriction threshold of an occupant, the coverlet utilizes sensors 24 and the heat withdrawal structure 44. As shown in FIG. 9, vasoconstriction of blood vessels leads to a decrease in perfusion of oxygen to the cells. Therefore, it is desirable to identify the point of vasoconstriction in each patient and avoid reaching such a level so to avoid decreased perfusion and maintain occupant comfort. This may be accomplished via the method shown in FIG. 8. The method includes determining the present heat withdrawal of the patient located on the patient support apparatus 12, at step 201. Step 202 includes monitoring the present heat withdrawal rate of the patient. The coverlet 26 is configured to determine when a patient is experiencing surface heat withdrawal at a rate of 0-10 W/m², at step 203. Step 204 includes increasing the heat withdrawal level in increments of approximately 20 W/m² upon confirmation that he patient is experiencing a surface heat withdrawal at a rate of 0-10 W/m² in step 203. At step 205, the coverlet 26 measures the skin temperature of the patient at each 20 W/m² increase. Such measurements are then recorded, at step 206. Step 207 includes comparing the measured skin temperatures of the patient and the heat withdrawal rate at each 20 W/m² increment to determine a slope. The sensors 24 and the heat withdrawal structure 44 determine if the slope if linear, at step 208. If so, the heat withdrawal structure 44 and sensors 24 return to step 204. If the slope has broken, then the breaking point is identified and recorded as the unique vasoconstriction threshold of the patient located on the patient support apparatus 12, at step 209. In approaching this break in linearity, the skin cools more rapidly due to the constriction of peripheral blood vessels as no more heat is carried to the skin by the blood. Therefore, the skin temperature of an occupant cools more rapidly at the heat withdrawal rate is increased and an occupant's skin temperature decreases for a given increment of surface heat withdrawal as the vasoconstriction threshold is approached and passed. As such, step 210 includes decreasing the heat withdrawal by 20 W/m². At step 211, the skin temperature of the patient is monitored as the heat withdrawal rate is lowered. The controller 80 may then determine if the skin temperature has decreased, at step 212. If so, then the heat withdrawal structure 44 returns to step 210 and decreases the heat withdrawal rate by 20 W/m². If not, then the system returns to step 211 and continues monitoring the skin temperature of the patient. The desired result of the method shown in FIG. 8 being an identification of the breaking point (i.e.: the break in slope linearity). Illustratively, such results may be measured and recorded similarly to those shown in FIG. 9 wherein the slope between the heat withdrawal rates of 13 w/m² to 63 w/m² is greater than that of the slope between the withdrawal rates of 63 w/m² to 168 w/m².

When a break in linearity is measured and recorded via the sensors 24 of coverlet 26, the controller 80 of the coverlet 26 is configured to decrease the rate of surface heat withdrawal so that the occupant experiences less cooling provided by coverlet 26 and maintains an optimal rate of perfusion as shown at step 210 of FIG. 8. The controller 80 of the coverlet 26 is further configured to measure, record, and adjust the rate of surface heat withdrawal automatically at periodic intervals as discussed above. These intervals may be programmed to run at intervals preprogrammed into controller 80, they may be randomly run by the controller 80, or some combination thereof.

As shown in FIG. 11, to increase or decrease the surface heat withdrawal, a user, such as a nurse or caregiver, may use the controller 80 to specify a desired total heat withdrawal rate. In response to the user's input, the controller 80 commands operation of the cooler 86 to chill the ambient air or the heater 90 to heat ambient air dependent upon the desired surface heat withdrawal input by the user.

As shown in FIG. 10, in some embodiments, a water removal system 88 may be included in the heat withdrawal structure 44 and configured to drain or otherwise remove the liquid from the coverlet 26. The illustrated water removal system 88 includes a nucleation device 110 to promote and enhance the efficiency of the transition from the gaseous phase to the liquid phase. One example nucleation device 110 is a device having an array of vertically oriented fibers 112 projecting into the airstream 74, as shown in FIG. 10. The fibers 112 converge into a funnel 114, and water droplets collect on the fibers 112. The weight of the water droplets causes them to migrate down the fibers 112 until eventually the water drips into the funnel 114 from the fibers 112, thereby channeling the water out of the heat withdrawal structure 44. The chilled, demoisturized air is then supplied to the inner chamber 56 of the coverlet 26 where its enhanced dry flux capacity and wet flux capacity are manifested as actual heat transfer.

Although this disclosure refers to specific embodiments, it will be understood by those skilled in the art that various changes in form and detail may be made without departing from the subject matter set forth in the accompanying claims. 

1. A method for controlling performance of a heat withdrawal coverlet for removing heat from a surface supporting a person, the coverlet having a flow path for guiding a stream of air along at least a portion of the surface, comprising: measuring heat withdrawal rate of the coverlet over time; identifying when the coverlet is withdrawing heat at first rate; increasing the heat withdrawal rate from the first rate between in predefined increments; measuring a skin temperature of an occupant at each increment of increase of the heat withdrawal rate; recording the skin temperature of the occupant at each increment of increase of the heat withdrawal rate; comparing the skin temperature of the occupant and the heat withdrawal rate increment to determine a slope of the relationship between the skin temperature of the occupant and the heat withdrawal rate; and determining the skin temperature and the heat withdrawal rate at which the slope breaks in linearity to determine a vasoconstriction threshold.
 2. The method of claim 1, wherein the relationship between the skin temperature of the occupant and the heat withdrawal rate is an inverse relationship such that as the heat withdrawal rate decreases, the skin temperature of the occupant increases.
 3. The method of claim 2, wherein an optimal heat withdrawal rate is located prior to a slope break, the optimal heat withdrawal rate configured to maximize the heat withdrawal from the occupant.
 4. The method of claim 3, wherein the optimal heat withdrawal rate determines the rate at which heat is withdrawn from the skin of an occupant prior to vasoconstriction.
 5. The method of claim 2, wherein the slope decreases after breaking such that the measurement of the slope before the break is greater than the measurement of the slope after the break.
 6. The method of claim 1 further comprising the steps of: decreasing the heat withdrawal rate after identifying the vasoconstriction threshold; waiting an amount of time so that the skin temperature of the occupant decreases and returns to a baseline temperature; identifying when the coverlet is withdrawing heat at a predefined rate; incrementally increasing the heat withdrawal rate from the predefined rate until the optimal heat withdrawal rate is reached; and maintaining the optimal heat withdrawal rate.
 7. A method for controlling performance of a heat withdrawal coverlet for removing heat from a surface supporting a person, the coverlet having a flow path for guiding a stream of air along at least a portion of the surface, comprising: measuring the skin temperature of a person on the coverlet; varying the operation of the coverlet to vary the heat withdrawal rate; monitoring the slope of the rate of change in skin temperature to the heat withdrawal to determine a change in linearity of the slope; using the heat withdrawal rate at the change in linearity to determine vasoconstriction threshold for the person; maintaining the heat withdrawal rate of the coverlet to operate below the vasoconstriction threshold of the person.
 8. An apparatus for controlling the heat withdrawal from a patient's skin comprising: a coverlet having an upper surface that is vapor permeable and air impermeable, the coverlet have inlet and an outlet and an interior space that provides a flow path for air to flow from the inlet to the outlet, the coverlet including a sensor for monitoring the heat withdrawal from the coverlet and a sensor for measuring the temperature of the skin of a person supported on the upper surface of the coverlet; an air treatment system having an inlet for admitting ambient air and an outlet for discharging treated air, a conduit connecting the outlet of the air treatment system to the inlet of the coverlet, and a controller including a processor and a memory device, the memory device including instructions that, when executed by the processor, cause the controller to monitor the sensors and operate the air treatment system to vary the heat withdrawal from the coverlet and monitor the skin temperature of the person to determine the slope of the rate of change in skin temperature to the heat withdrawal to determine a change in linearity of the slope, use the heat withdrawal rate at the change in linearity to determine vasoconstriction threshold for the person, and maintain the heat withdrawal rate of the coverlet to operate below the vasoconstriction threshold of the person.
 9. The apparatus of claim 8, wherein the air treatment system includes a blower under the control of the controller to vary the heat withdrawal.
 10. The apparatus of claim 8, wherein the air treatment system includes a cooler for cooling the ambient air under the control of the controller to vary the heat withdrawal.
 11. The apparatus of claim 8, wherein the air treatment system includes a heater for heating the ambient air under the control of the controller to vary the heat withdrawal.
 12. The apparatus of claim 8, wherein the air treatment system includes a water removal system for removing water from the ambient air under the control of the controller to vary the heat withdrawal.
 13. The apparatus of claim 8, wherein the air treatment system includes a valve assembly for controlling the flow of the air out of the air treatment system under the control of the controller to vary the heat withdrawal.
 14. The apparatus of claim 8, wherein the memory device further includes instructions that, when executed by the processor, cause the controller to, decrease the heat withdrawal rate after identifying the vasoconstriction threshold; wait an amount of time so that the skin temperature of the occupant decreases and returns to a baseline temperature; identify when the coverlet is withdrawing heat at a predefined rate; incrementally increase the heat withdrawal rate from the predefined rate until the optimal heat withdrawal rate is reached; and maintain the optimal heat withdrawal rate.
 15. The apparatus of claim 14, wherein the air treatment system includes a blower under the control of the controller to vary the heat withdrawal.
 16. The apparatus of claim 14, wherein the air treatment system includes a cooler for cooling the ambient air under the control of the controller to vary the heat withdrawal.
 17. The apparatus of claim 14, wherein the air treatment system includes a heater for heating the ambient air under the control of the controller to vary the heat withdrawal.
 18. The apparatus of claim 14, wherein the air treatment system includes a water removal system for removing water from the ambient air under the control of the controller to vary the heat withdrawal.
 19. The apparatus of claim 14, wherein the air treatment system includes a valve assembly for controlling the flow of the air out of the air treatment system under the control of the controller to vary the heat withdrawal. 